Assessing Fe and Zn Content in Egeria densa: Sample Treatment, Spatio-Temporal Distribution, and Wetland Herbivory Implications

Wetlands are delicate ecosystems that host diverse species and face ongoing environmental stress. The “Carlos Anwandter” Ramsar Site in Valdivia, Chile, is the world’s main breeding ground for the black-necked swan, which strongly relies on the aquatic plant Egeria densa. This area has been impacted by anthropogenic activities that have increased particulate iron (Fe) and zinc (Zn) deposition. However, standard protocols for metal analysis encourage eliminating any particles on the plant’s surface, neglecting the contribution of deposited particulate contaminants. Appropriate sample treatment is therefore essential to quantify metal concentrations and the potential impact on herbivore species. This study aimed to evaluate how sample treatments and plant sectioning affect Fe and Zn concentrations in E. densa. Samples were collected from both the Ramsar site (Cruces River) and a control site (Calle-Calle River). Results showed that washing samples (both in the field and lab) significantly reduced reported metal concentrations, underscoring the importance of standardised sampling and pre-treatment protocols. Fe concentrations were notably higher at the Ramsar site (11,155 mg kg−1) compared to the control (3783 mg kg−1). The same is true for Zn (108 mg kg−1 and 60 mg kg−1, respectively). Over time, Fe concentrations remained stable, while Zn concentrations declined, suggesting a consistent Fe input and a decreasing Zn trend in the wetland. These findings are crucial for interpreting metal pollution and understanding spatial–temporal variability in aquatic plant contamination.